Combustion air preheating systems

Another very important quality characteristic is the combustion efficiency (fuel efficiency) of radiant tubes. The efficiency strongly depends on the flue gas outlet temperature of radiant tube systems. In order to achieve maximum efficiency, air preheating systems use the enthalpy of the flue gas for preheating the combustion air simultaneously reducing the flue gas outlet temperature of such systems. High efficiencies with less flue gas losses result in lower fuel consumptions and lead to a reduction of the CO2 emission per ton of product. 

A typical combustion air preheat temperature for this type of plant is approximately 750°F. This permits use of carbon steel ducting in the air preheat distribution system. Table 4 gives a typical feed and utility summary for a limited export steam plant, as defined above. 

Burners and combustion air ports are located in the walls of the furnace to introduce either heat or air where needed. The air path is countercurrent to the sohds, flowing up from the bottom and across each hearth. The top hearth operates at 310—540°C and dries the feed material. The middle hearths, at 760—980°C, provide the combustion of the waste, whereas the bottom hearth cools the ash and preheats the air. If the gas leaving the top hearth is odorous or detrimental to the environment, afterburning is required. The moving parts in such a system are exposed to high temperatures. The hoUow central shaft is cooled by passing combustion air through it. 

Chemical Regeneration. In most MHD system designs the gas exiting the toppiag cycle exhausts either iato a radiant boiler and is used to raise steam, or it exhausts iato a direct-fired air heater and is used to preheat the primary combustion air. An alternative use of the exhaust gas is for chemical regeneration, ia which the exhaust gases are used to process the fuel from its as-received form iato a more beaeftcial oae. Chemical regeaeratioa has beea proposed for use with aatural gas and oil as well as with coal (14) (see Gas, natural Petroleum). 

The success of preheater kiln systems led to precalciaer kiln systems. These units utilize a second burner to carry out calciaation ia a separate vessel attached to the preheater. The flash furnace (57), eg, utilizes preheated combustion air drawn from the clinker cooler and kiln exit gases and is equipped with an oil burner that bums about 60% of the total kiln fuel. The raw material is calciaed almost 95%, and the gases continue their upward movement through successive preheater stages ia the same manner as ia an ordinary preheater. 

The hot combustion gas preheats the fresh air and the prereformer, and can be used further to generate steam. The system is cooled with 200 to 300 percent excess air, A 25-kW S()F(i generator system is shown in Fig, 27-69,... 

Economizer corrosion rates are enhanced by higher heat-transfer rates excessive heat flux may create localized nucleate boiling zones where gouging, as a result of chemical concentration effects, can occur. Air heaters are also located in the exit gas system. They do a job similar to that of economizers except that they preheat combustion air. 

An heat exchanger located in the exit-gas system. Air heaters preheat combustion air and may be of several different types including convection air heaters of either tubular or plate design and regenerative air heaters. 

Natural gas systems with endothermic steam reformers often make use of the residual fuel from the anode in a reformer burner. Alternatively, the residual fuel could be combusted prior to a gas expander to boost performance. In an MCFC system, the residual fuel often is combusted to maximize the supply of CO2 to the cathode while at the same time providing air preheating. In an SOFC system, the residual fuel often is combusted to provide high-temperature air preheating. 

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